Tommi can't grab a dagger. The hobgoblin captain probably has one, but Gutter IS Shorgg's pocket knife. Drawing Gutter would take up his "free object interaction". Tommi can use it with two-weapon fighting, though. Like I said, (maybe I make it confusing with my description!). Technically, I imagine it as an oversized knife, but rules-wise it would count as a scimitar, I guess, in that it is a 1d6 slashing light finesse martial weapon. That works for 2WF just as good as a shortsword.
Oh! Your worry is that Rogues are not proficient in scimitars. That is WEIRD.
Well, anyway, I'll certainly allow Tommi to use it. Heck, if Tommi's training is all about thrusting motions, I can't see any reason why it couldn't be a piercing weapon and just count as a shortsword. (Fluff-wise, it's only bladed on one edge, but there's no reason you can't stab with it. It's pointy!)